This invention relates to a method for preparing cathodes for use in scanning electron microscopes, and more specifically, this invention relates to a method for preparing single crystals of LaB.sub.6 suitable for use as cathodes in electron guns in scanning electron microscopes (SEM) or scanning transmission electron microscopes (STEM).
In electron microscopes, the specimen under study is illuminated by focusing a beam of electrons on it with a system of magnetic lenses. The final image is then made visible by means of a florescent screen, or it can be photographed directly. In a scanning electron microscope, the electrons are focused down to a very fine beam which is then scanned across the surface of the specimen by pairs of magnetic coils. The electrons are collected in an electron detector and the final image is ultimately displayed on a cathode ray tube (CRT); the image being built up on the screen in synchronism with the scanning movement of the initial electron beam on the specimen surface. In the SEM, resolution directly depends upon the diameter of the scanning beam, its depth of penetration and upon the signal-to-noise ratio. For high resolution, the diameter of the scanning primary electron beam must be kept as small as possible. The current of this beam must be kept as high as necessary at the same time to have a good signal-to-noise ratio. In general, the source of electrons for this beam is provided by a hairpin-shaped cathode made of tungsten. The development of lanthanum hexaboride cathodes as a replacement for the tungsten hairpin filament in electron microscopes and similar devices has been described in the literature, Journal of Applied Physics. 38, 1991-2 (1967) A. N. Broers. In a subsequent report, Journal Phys. E. 2 273-6 A. N. Broers, a cathode electron gun using the LaB.sub.6 cathode was described, which could, under proper operating conditions, provide brightness 15 to 20 times that of tungten cathodes operated at 40 hour life conditions.
The lanthanum hexaboride cathodes presently available are polycrystalline in structure, about 1 mm square and generally prepared by hot pressing and sintering highly purified lanthanum hexaboride powder to achieve a density on the order of 90 to 95% of the theoretical value. The tip of the cathode is then ground to a tip radii of from 4 to 12 .mu.. It is known in the art that an improvement in brightness is obtained if one maintains the radius of curvature at the cathode tip as small as possible. Recent studies have shown that the emission of electrons from lanthanum hexaboride cathodes occurs at localized regions, called lobes, and that high brightness may be obtained only by correct positioning of these lobes in the electron column. The origin of these lobes is at present uncertain but it is believed that they are single crystals of lanthanum hexaboride which become prominent when the crystalline material evaporates from the grain boundaries at a faster rate than from the grains themselves so that an irregular shaped protrusion is formed on the top of the electrode. Since, as discussed previously, the flow of electrons emanates from the tip of the cathode, a single protrusion or lobe at the tip, if the radius is small, will generally cause little or no decrease in brightness. However, if as it often happens, two or more lobes are formed at the tip, the flow of electrons emanates from two or more of the lobes and the overall brightness of the electrode is decreased substantially with a corresponding decrease in microscope capability. While it is obviously possible to regrind the cathode tips to remove the offending lobes, such effort is time consuming since removal of cathodes from microscopes is difficult, as is the precise grinding of the tips. Thus, such removal for regrinding is expensive and does not always solve the problem.
One solution to the problem is the preparation of cathodes from single crystals which would offer greater control of lobe formation because of better control of structure and composition in single crystals. The preparation of single crystals of LaB.sub.6 has been reported in Materials Research Bulletin, Vol. 1, 1966, pages 27-31, by a technique of zone melting using an induction heated zone melting apparatus. However, this technique requires very expensive equipment and produces large crystals from which individual cathodes must be machined.